To minimize the need for human surveillance, and to provide a back up therefore, vibration sensors are in widespread use on fencing such as International borders between States, perimeter fencing around military instalments, prisons and the like. Such sensors give real time indication of attempts to scale or to cut through the fence, and are required to give years of maintenance free service in all weathers.
Such vibration sensors typically comprise a conductive ball supported on three mutually insulated contact supports mounted on a signal cable, such that sudden vibrations displace the ball and momentarily break the circuit sending a signal along the cable. Electronic means can filter out random noise, the effects of wind, birds and the like, and analysis of the frequency of the pulses and their duration can provide an indication of the probable cause.
For optimal performance, vibration sensors of this type are required to be mounted in a perfectly horizontal plane, but particularly along international borders, that follow the relief of the natural terrain, such sensors are fixed to wire fencing that is often situated in hilly, difficult to get to places. Although it will be appreciated that long, maintenance free life is a basic requirement for such sensors. Frequently such sensors are exposed to the elements. One major development that overcame many of the mounting problems and provided protection against the elements was the mounting construction for a motion sensor described in U.S. Pat. No. 4,107,545 to Gittelis, which describes a substantially spherical sensor casing constructed from two substantially hemispherical parts affixed to a complementary socket portion by a clamping means. This construction allows two degrees of freedom, enabling precise angular orientation, of sensor with respect to the fencing, and enabling the ball mounts to be oriented horizontally.
In practice, sensor housings and mounting constructions in accordance with '545 to Gittelis typically included two sensors per housing, mounted in series, the second sensor providing a degree of backup and reliability. Each sensor ball and its support legs were gold plated to provide tarnish free, very low resistance contacts, having resistances of perhaps 0.1-0.2 ohm, and allowing many sensors to be attached in series along large stretches of fencing. However, each vibration dislodged the ball from its tripod like, three-point support, and caused wear in the coating. Similarly, spiking from lightening, and even fluctuations in the signal carrying current over the course of time, resulted in damage to the gold plating, and, to provide long term functioning, the second sensor within the housing was an important backup.
In practice, to provide 8-10 years of trouble free use, each section of fencing, i.e. each span of fencing between adjacent support posts, required two sensor housings, mounted about 0.5 m apart. This was to provide, as a backup to the first sensor housing, an additional safety layer; it being appreciated that a single housing could be intentionally rotated out of the horizontal plane by a terrorist infiltrator or inadvertently displaced by wildlife, such as a bird, for example. Furthermore, if an individual sensor unit (housing containing sensors) were intentionally bypassed by carefully shorting it out, the second sensor unit would still work. Finally, having two sensor units provided four sensors, which gave a high level of reliability. Care was taken that the signal cable connecting the two sensor units was slack, so that each sensor unit worked independently.
The arrangement of two sensor units per stretch of fencing, each containing two sensor balls in series, proved itself by giving reliable service over many years in a variety of weather conditions, and such sensor units became standard along Israeli security fences and International borders, around military installations such as airfields, around factories and prisons in Europe, and around presidential palaces in and prime minister's offices in various countries.
Despite the high reliability of the housings described in '545 to Gittelis, over the years, several shortcomings were noted: Firstly, despite having a drainage hole in the lowest point of the housing, due to the clamping means that covered the hole, moisture seeping into the housing through the cable entry and exit holes, perhaps rain drops running along the cable, sometimes did not adequately drain out through the drainage hole, and such internal moisture sometimes resulted in shorting between contacts, preventing displacement of ball from providing a detectable signal. Secondly, occasionally woodpeckers and magpies drilled holes through the plastic housing, exposing sensor to elements, and sometimes removing the ball. Thirdly, although two balls provide a degree of backup, occasionally the gold contact layer scraped through in both sensors, disconnecting the signal. Fourthly, since gold plated balls having negligible resistance were used, occasionally a clever terrorist using a shorting wire bypassed sensors, and such sabotage sometimes proved very difficult to detect. Fifthly, it will be appreciated that having to mount two sensor housings per section rather than one, significantly increases capital investment, partly due to the unit cost per sensor housing, but mostly being a function of the significant manpower required for correct installation of the sensors and their orientation into the horizontal plane. Sixthly, in transport and prior to installation, the sensor balls suffered a degree of wear due to bouncing about and scraping against mounting supports during transportation, which lowered their useful working life.
Thus, despite the usefulness of the sensor housings described in U.S. Pat. No. 4,107,545 to Gittelis, incorporated herein by reference, there is a need for an improved sensor housing, having novel features that overcome these disadvantages, and the embodiments of the present invention provide such an improved sensor housings.